1. Field of the Invention
The present invention relates to a phased array radar tracking system. For radar technical terms in the following description a basic textbook is recommended, e.g. S.Kingsley and S.Quegan, Understanding Radar Systems, McGraw Hill, 1992.
2. Description of the Related Art
The proposed radar design is intended for tracking aircraft targets. The radar system has a phased array antenna, which means that it can be controlled and directed electronically. Each detected aircraft target is followed and represented as a track. The track is a state vector with elements for a set of parameters. The main way of working for the radar is to transmit pulses with a certain pulse repetition frequency and carrier frequency, in a certain direction. After being reflected against a radar target (subsequently called "the target") they may be measured by a receiver. The time delay from transmission to reception of a pulse is proportional to the target distance.
The distance is, however, ambiguous since the pulse frequency value is so high that several pulses are transmitted before the reflection of the first pulse returns. This ambiguity gives rise to an ambiguity problem in calculation of the distance: Each measured time between transmission and reception of a pulse corresponds to several possible ranges. The two-way distance that a certain radar pulse can go in the time interval between two consecutive pulses is called the range-unambiguity interval. The length of the range-unambiguity interval depends on the value of the pulse repetition frequency (PRF). For a radar of this type the number of selectable PRF values usually amounts to some tens. A sequence of pulses transmitted with a certain PRF is called a pulse train. Between each pair of adjacent range-unambiguity intervals there is a blind region dependent on the fact that it takes a certain time to transmit the pulse from the antenna.
Before each new measurement of a target the position of the target as well as the position uncertainty is predicted. A common computation technique for this prediction is Kalman filtering. The position uncertainty forms an uncertainty volume (or uncertainty-region), which grows roughly quadratically with the time since the latest measurement. In order to master both the uncertainty about the target position and the radar-target range-unambiguity, it is necessary that the extension of the uncertainty volume--along the radius between target and radar--is contained completely in one single range-unambiguity interval. Due to the (predicted) target movement in relation to a possibly moving radar this condition is satisfied only during some limited time intervals, namely such time intervals for which the radar-target range and the position uncertainty region for the target lies completely within the limits of one single range-unambiguity interval, depending on the selected PRF value.
How this PRF value, these time intervals and the time point for measurement shall be calculated is one of the problems that have to be solved in a scheduling device of a phased array radar. Several other factors must however also be considered in this calculation.
One factor is associated with a combination of Doppler frequency shifts and ground echo cancellation. If a movement of a target between two pulses in the pulse train equals in radial direction a number of half wave lengths for the carrier wave, the target seems to stand still. For each PRF a number of (equally large) speed unambiguity intervals arises, during which the target speed is unambiguous. Furthermore, all echoes from slow targets must be cancelled (ground echo cancellation). The combination of these two effects leads to "blind spots" in the speed spectrum. This phenomenon is called Doppler blindness.
A third problem occurs as the radar can measure only one target at each instance. For each track to be scheduled, the measurement time intervals assigned to other tracks are already occupied.
A fourth problem is caused by so called "cross-overs": If the position uncertainties of more than one target during some time interval lie in beam sectors that overlap this time interval will be impossible to use for measurement.
The common thing with all these problems is that they depend on the choice of PRF value and scheduled measurement time interval. The problem complexity may grow as further conditions may have to be added on the radar. The problem complexity is a new one as phased array radars are new. Known approaches to solve the problem handle one track at a time by first assigning to it the next free time period, and then calculating--if possible--the PRF value that fulfils all demands. In another approach it is determined which tracks need to be measured each time the radar is "free", which of these tracks that it is most necessary to measure and then a PRF value for this track is calculated.